Thymidylate synthetase from hog thymus glands will be purified using 5-fluorodeoxyuridylate affinity chromatography. Attempts will be made to show that the formation of a covalent intermediate is involved in catalysis by this enzyme. Groups at the active site responsible for the formation of this intermediate will be identified. Studies on the chemical mechanisms of the thiol and SO3 promoted dehalogenation of 5-bromo- and 5-iodouracil will be continued. Work recently initiated on the enzymatic dehalogenation of the halouracils will be continued.